DEMO MANUAL DC2176A LT3909 2-String 60mA, 2MHz Step-Up LED Driver with 2% Current Matching Description Demonstration circuit 2176A features the LT 3909, a is pulled up by 100k R6 to V , but there is a placeholder IN 2-string 60mA, 2MHz step-up LED driver with 2% to pull up FAULT to INTV instead. CC current matching. Resistor divider R4 and R5 from the input to the EN/UVLO The LT3909 precisely regulates two LED strings for equal pin programs the circuit to turn off when V decreases IN current, supports LED dimming that reduces brightness below 5V with 0.9V of hysteresis. The circuit can also be down to a 1/40,000 ratio, protects against faults and also turned off by connecting the EN/UVLO terminal to ground. reports faults. The LT3909 consists of a step-up converter An external voltage must be applied to the PWM terminal with a fixed 2MHz switching frequency and current sources in order for DC2176A to operate. The external voltage can for the LED strings. be a DC level or an appropriate PWM dimming signal. A The operating voltage range for DC2176A is from 7V to common frequency for PWM dimming is 100Hz. There is 36V. The maximum current in each LED string is 40mA a placeholder for a resistor to pull up PWM to INTV so CC and is programmed by 24.9k I resistor R1. The I an external voltage on PWM for operation is unnecessary. SET SET resistor can be changed to 20k or 16.7k to program the There is a placeholder that allows an external diode to string current to 50mA or 60mA respectively. 16.7k is a be placed in parallel with the internal LT3909 rectifier for 0.1% resistora 1% 16.9k resistor will result in slightly higher current capability. DC2176A uses an inductor that less than 60mA string current. See the data sheet for ap- is optimized for high efficiency, and the input supply for plication examples with 50mA and 60mA string currents. power to the inductor can be separated from the LT3909 The LT3909 utilizes voltage feedback during PWM dim- V pin to allow operation from two input sources. Ceramic IN ming and when there are faults on both LED strings. Volt- capacitors are used at the input, output and for INTV . A CC age feedback resistors R2 and R3 on DC2176A optimize 50V tantalum polymer capacitor protects the input from operation for LEDs that are 35V maximum. Efficiency is ringing that may occur when the input is hot-plugged using 86% at 12V input, 35V for the LED string forward voltages long leads. There are placeholders occupied by 0805 size and 40mA per string. 0 jumpers on the back of the PCB for two LED strings and also placeholders for SOD323 diodes to clamp the LED current can be reduced by an external voltage applied LED1 and LED2 voltages from ringing below ground. to the CTRL terminal. Resistor R8 pulls up CTRL to the INTV voltage of the LT3909 so the LED current is at the 2-string operation, close current matching, an extremely CC 40mA maximum when there is no external CTRL voltage wide dimming range and fault protection make the LT3909 applied. There are placeholders to add voltage dividers ideal for functionally advanced back lighting and illumi- either from V or INTV to the CTRL pin. A voltage nation circuits. DC2176A features the LT3909EDD in a IN CC divider from V to the CTRL pin can be used to reduce thermally enhanced 12-lead 3mm 3mm DFN package. IN LED current at low input voltages. A resistor divider from The LT3909 data sheet must be read in conjunction with INTV to CTRL allows an NTC to reduce LED current at this demo manual to properly use or modify DC2176A. CC high temperatures. There are placeholders for an RC filter Design files for this circuit board are available at between the CTRL terminal and the CTRL pin. The RC filter DEMO MANUAL DC2176A p s Specifications are at T = 25C A PARAMETERS CONDITIONS MIN TYP MAX UNITS Minimum Input Voltage, V V = V = 35V, I = I 40mA 7 V IN LED1 LED2 LED1 LED2 Maximum Input Voltage, V V = V = 35V, I = I 40mA 36 V IN LED1 LED2 LED1 LED2 Maximum LED Voltage, V , V R2 = 976k 1%, R3 = 31.6k 1% 35 V LED1 LED2 LED Current, I , I R1 = 24.9k, V = INTV 40 mA LED1 LED2 CTRL CC Input EN/UVLO Turn-On Voltage, V R4 = 402k, R5 = 130k, V Rising 5.9 V IN IN Input EN/UVLO Turn-Off Voltage, V R4 = 402k, R5 = 130k, V Falling 5 V IN IN Efficiency V = 12V, V = V = 35V, I = I = 40mA 86 % IN LED1 LED2 LED1 LED2 Switching Frequency 2 MHz Quick s p D To evaluate the performance of the LT3909 using DC2176A, 4. Apply an external voltage to the PWM terminal. The refer to Figure 1 for the proper measurement equipment external voltage can be a 2V to 5V DC source or a 100Hz setup and then follow the procedure below: or higher PWM dimming signal. Install resistor R10 to pull up PWM to INTV if an external voltage on PWM CC NOTE: An external voltage must be applied to the PWM is not desired. terminal in order for DC2176A to operate. See Step 4 for details. 5. Turn the V power supply on and verify that the input IN voltage is between 7V and 36V. The V voltage is al- IN 1. Connect two strings of LEDs with a forward voltage of lowed to exceed the LED voltage by a few volts, but 35V or less and corresponding meters. a V that is much higher than the LED voltages may IN 2. Connect the EN/UVLO terminal to GND with a clip-on activate overtemperature protection or dissipate too lead. much power in the LT3909. 3. With the power off, connect the input power supply and 6. Release the EN/UVLO to GND connection. corresponding meters. 7. Once the LED strings are running at the programmed LED current adjust the input voltage within the operat- ing range and observe the LED current regulation or reduction in brightness with LED dimming. dc2176afa 2 ure roce tart ummary erformance